1. Field of the Invention
This invention relates to a control system for controlling a power steering apparatus which controls the steering assist amount of a steering mechanism of a vehicle, for example, of a type wherein the target assist amount is set in accordance with a fuzzy logic rule.
2. Description of the Related Art
In recent years, power steering apparatuses have varied widely for assisting the force (hereinafter referred to as steering wheel operating force or steering force) for operating a steering wheel. Particularly, hydraulic power steering apparatus which make use of a hydraulic cylinder mechanism to hydraulically assist the steering wheel operating force are popularly employed as such power steering apparatuses. Also electrically operated power steering apparatuses wherein the steering wheel operating force is assisted by an electric motor have been developed.
Such power steering apparatuses as described above allow steering of a vehicle, for which high steering wheel operating force is normally required, such as, for example, in a large size vehicle or a vehicle which employs wide tires or wheels which must be steered, to be performed with a low steering wheel operating force, thereby eliminating the so-called heavy operation of the steering wheel. When the vehicle speed is low such as upon garaging, generally the steering operation is desired to be performed with a lower steering force. On the other hand, when the vehicle is running at a high speed, if the steering operation is very light (sensitive to a low amount of force), then running of the vehicle becomes unstable. Accordingly, in such situations, the steering operation is desired to be heavy (sensitive to a high amount of force). Thus, a vehicle speed responsive power steering apparatus has been developed wherein the steering wheel operation is controlled in response to the speed of the vehicle. Thus, when the vehicle runs at a low speed, the steering assist amount is set to a comparatively high value so as to make the steering wheel operation lighter. Further, when the vehicle runs at a medium or high speed, the steering assist amount is set to a comparatively low value to make the steering wheel operation heavier.
In one such vehicle speed responsive power steering apparatus, a vehicle speed sensor is provided on the vehicle while a valve for adjusting hydraulic oil to be supplied to a power cylinder is provided in a hydraulic system of the hydraulic power steering apparatus. Operation of the valve is electronically controlled in response to a vehicle speed detected by the vehicle speed sensor to adjust the steering assist amount. The vehicle speed responsive power steering apparatus of the type just mentioned is called an electronically controlled power steering apparatus.
FIG. 16 is a schematic view showing construction of a hydraulic pressure control unit for a conventional electronically controlled power steering apparatus. FIG. 17 is a schematic cross sectional view taken along line XVII--XVII in FIG. 16. Finally, FIG. 18 is a schematic cross sectional view taken along line XVIII--XVIII in FIG. 16.
Referring to FIGS. 16 to 18, the numeral 11 indicates an input shaft for receiving steering force from the steering wheel (not shown), which is rotatably supported in a casing 12 by means of bearings. A pinion gear 13 is mounted for relative rotation at a lower end of the input shaft 11 with a bushing or the like (not shown) interposed therebetween. A torsion bar 14 is located in the hollow inside of the input shaft 11. The torsion bar 14 is coupled at an upper end thereof for integral rotation to the input shaft 11 by way of a pin or the like. Further, it is not restrained at a lower end thereof by the input shaft 11.
The pinion gear 13, at the lower end of the input shaft 1, is held in serration coupling engagement with the lower end of the torsion bar 14 so that the steering force inputted to the input shaft 11 may be transmitted to the pinion gear 13 by way of the torsion bar 14. The pinion gear 13 is held in meshing engagement with a rack 15 so that the steering force of the input shaft 11 may be transmitted to the rack 15 by way of the pinion gear 13 to move the rack 15 in its axial direction (in a direction perpendicular to the plane of FIG. 16) to steer wheels of the vehicle (not shown).
In the casing 12, a rotary valve 16 is disposed between the input shaft 11 and the pinion gear 13. The rotary valve 16 is opened and closed in response to a difference in phase between the input shaft 11 and the pinion gear 13. The rotary valve 16 is connected with a hydraulic oil supply tube 18 of an externally provided oil pump 17 and a hydraulic oil discharge tube 20 of an oil reservoir 19. 0n the other hand, the numeral 21 indicates a power steering hydraulic cylinder, wherein a piston 23 is supported to be movable in an axial direction in a hollow cylinder 22 provided on a predetermined member on the vehicle body side. A piston shaft 24 of the piston 23 is mounted halfway on the rack 15. The piston 23 partitions the inside of the cylinder 22 into two parts to form right and left oil chambers 25 and 26.
Therefore, when steering force is inputted to the input shaft 11, the input shaft 11 is rigid and presents little distortion, but the torsion bar 14 transmits the steering force to the pinion gear 13 while presenting some distortion. Consequently, the pinion gear [3 presents a difference in phase with respect to the input shaft 11 towards the steering side, and the rotary valve 16 is driven according to the difference in phase. The rotary valve 16 is opened and closed so that hydraulic oil is supplied from the oil pump 17 through the hydraulic oil supply tube 18 to the right and left oil chambers 25 and 26 of the hydraulic cylinder 22 to provide the rack 15 with a steering assist force, producing required steering assist force in the steering direction.
Further in the casing 12, a plurality of reactive force plungers 27 for providing, upon steering, steering reactive force to increase the steering force (that is, steering reaction) are provided on an outer periphery of a lower portion of the input shaft 11 such that they surround the outer periphery of the input shaft 11. The reactive force plungers 27 receive hydraulic oil supplied thereto under the control of a hydraulic pressure control valve 28 to restrain the input shaft 11 to provide steering reactive force in response to the hydraulic pressure.
Specifically, four reactive force plungers 27 are provided at equal intervals in the casing 12 so as to surround the outer periphery of the input shaft 11 and, at its outer end side, chambers 29 are formed and return orifices 30 are provided. On the other hand, the hydraulic pressure control valve 18 is provided in the casing 12 laterally adjacent and parallel to the input shaft 11. In the hydraulic pressure control valve 28, a spool 31 is provided to be movable in a vertical direction in the casing 12, which is biased downward by a spring 32 provided at an upper part. A solenoid 33 is provided on an outer periphery of lower part of the spool 31 so that the solenoid 33 is energized to exert upward axial force to the spool 31.
The spool 31 has a pair of oil passages 34 and 35 communicated with the hydraulic oil discharge tube 20 of the oil reservoir 19, an annular oil passage 36 for communicating with the hydraulic oil supply tube 18 of the oil pump 17, and an annular oil passage 38 for communicating with the chamber 29 of the reactive force plunger 27 by way of a hydraulic supply/discharge tube 37, and an oil passage 39 for communicating the annular oil passages 36 and 38 with each other. Therefore, normally, when the solenoid 33 is unenergized, the spool 31 is at its downward position, and the hydraulic oil supply tube 18 and the annular oil passage 36 communicate with each other. As a result, hydraulic oil supplied to the hydraulic pressure control valve 28 through the hydraulic oil supply tube 18 is supplied to the chamber 29 of the reactive force plunger 27 from the annular oil passage 36 through the oil passage 39 and the annular oil passage 38. On the other hand, when the solenoid 33 is energized, the spool 31 is at its upward position, and the hydraulic oil supply tube and the annular oil passage 36 do not communicate with each other. Therefore, hydraulic oil supplied from the oil pump 17 through the hydraulic oil supply tube 18 to the hydraulic pressure control valve 28 is not supplied to the chamber 29 of the reactive force plunger 27.
Thus, current applied to the solenoid 33 can be adjusted to control steering assist characteristics. Further, a control unit (CU) 40 for controlling the solenoid 33 is connected with a vehicle speed sensor 41, an engine speed sensor 42, and the like so that the control unit 40 sets the current applied to the solenoid 33 in response to output. signals from these sensors to control the solenoid 33.
Upon steering, for example, while the vehicle stops or is running at a low speed, maximum current is supplied to the solenoid 33. Consequently, the spool 31 is moved upwardly to its highest position in which the annular oil passage 36 is not communicated with the oil pump 17 and supply of oil to the chambers 29 of the reactive force plungers 27 is stopped. Consequently, the reactive force plungers 27 do not restrain the input shaft 11, and steering can be performed with light-force.
On the other hand, for example, while the vehicle is running at a medium or high speed, the current supply to the solenoid 33 is decreased in response to an increase of the vehicle speed. Consequently, when the steering wheel is at its neutral position, the axial force of the spool 31 decreases as the current decreases. Further, as the axial force decreases, the spool 31 is moved down so that the annular oil passage 36 is communicated with the hydraulic oil supply tube 18 of the oil pump 17 to allow oil to be supplied to the chambers 29 of the reactive force plungers 27. In this condition, the reactive force plungers 27 restrain the input shaft to hold the steering wheel at its neutral position. Then, if the steering wheel is moved a little from the neutral position, then the output of the oil pump 17 attempts to rise. In this instance, the discharging pressure of the oil pump 17 acts upon the chambers 29 of the reactive force plungers 27 almost without being controlled by the hydraulic pressure control valve 28. Accordingly, in the vicinity of the neutral position of the steering wheel, the steering force is increased and a sufficient response of the steering wheel at the neutral position is obtained. This results in a feeling of stability of the steering wheel in the neutral position.
Upon steering while the vehicle is running at a medium or high speed, the output of the oil pump 17 rises, within an ordinary steering range, to increase the steering assist amount in response to steering of the steering wheel, that is, in response to an increase of the steering force. Meanwhile, the discharging pressure of the oil pump 17 acts upon the chambers 29 of the reactive force plungers 27 while being controlled by the hydraulic pressure control valve 28. Accordingly, the reactive force plungers act to restrain the input shaft 11 to increase the steering response (steering force).
As a result, upon steering when the vehicle runs at a medium or high speed, the steering force is increased by an amount corresponding to the action of the reactive force plungers 27 as compared with the steering force acting upon steering when the vehicle stops or is running at a low speed. In short, the steering response is increased and a stable steering feeling is obtained. Particularly, when the current supply to the solenoid 33 is decreased in response to an increase of the vehicle speed, the steering assist amount decreases and the steering force (steering response) increases, and consequently, a more stable steering feeling can be obtained.
The control unit 40 for controlling the solenoid 38 is connected with the vehicle speed sensor 41 and the engine speed sensor 42 so that when trouble with a detection circuit is detected from vehicle speed information, an engine speed signal or the like, the solenoid 33 is turned off to effect fail-safe control.
Another consideration is that the required steering force characteristics actually vary depending upon a running condition of the vehicle, that is, whether the vehicle is running straightforward or along a curve, or whether the vehicle is being accelerated or braked. However, conventional electronically controlled power steering apparatuses have not been successful in always providing an optimum steering feeling since they merely control the steering force in response to the speed of the vehicle as described above.
For example, to assure steering stability in high speed running condition of the vehicle, the steering force characteristic is desired to present a somewhat heavy steering force with a decreased steering assist amount. However, in some cases, light steering with an increased steering assist amount may be advantageous for the driver even when the vehicle is running at a high speed.
That is, on a moderate and long curve with a large curvature or an entrance and exit of an expressway, the vehicle is required to hold the steering wheel at an appropriate steering angle according to the curvature of the road. In this case, if the steering assist amount of the vehicle is set to a small value and the steering force is somewhat heavy, the driver is required to use a high steering force (steering holding force), which is a heavy burden to the driver.
In addition to the electronically controlled power steering apparatus described above, several power steering apparatus have been proposed including a power steering apparatus disclosed in Japanese Patent Laid-open Publication 2-171384 wherein the steering assist amount is varied in accordance with a fuzzy logic rule from a steering direction signal of the steering wheel and a height signal of the vehicle. Another power steering apparatus is disclosed in Japanese Patent Laid-open Publication 2-171385 wherein the steering assist amount is varied in accordance with a fuzzy logic rule from a steering direction signal of the steering wheel and a temperature signal of the vehicle.
However, also those power steering apparatuses fail to control the steering assist-amount for the steering wheel so as to provide an optimum steering characteristic in response to a running condition of the vehicle and particularly do not always provide an optimum steering feeling, as described above, when the vehicle is running at a high speed on a moderate and long curve with a large curvature where the steering wheel must be held at a certain steering angle.
The present applicant has already applied for a patent on a fuzzy logic control type electronically controlled power steering apparatus which solves the above problems as Japanese Patent Application 4-334617 (filed in the U.S. on Dec. 15, 1992).
This fuzzy logic control type electronically controlled power steering apparatus has a target steering assist amount setting device for setting a target steering assist amount during electronic control, which sets the target steering assist amount in accordance with a fuzzy logic rule using the running speed and the level of a steering holding condition in response to the steering angle as input conditions. Specifically, the target steering assist amount setting device uses a first membership function For evaluating the running speed of the vehicle and a second membership function for evaluating the level of the steering holding condition in response to the steering angle to set the target assist amount in accordance with a fuzzy logic rule wherein the target assist amount is increased as the level of the steering holding condition increases. The level of the steering holding condition is determined from the vehicle speed, the steering angle, and the number of times the steering angular velocity changes in a unit time.
However, this fuzzy logic control type electronically controlled power steering apparatus fails to obtain a sufficient ease of steering when the vehicle runs at a high speed. That, is, in the fuzzy logic control type electronically controlled power steering apparatus described previously, the steering angle is applied as one of the factors for setting the level of the steering holding condition, a steering holding adaptation (level of the steering holding condition) is increased as an absolute value .vertline.ha.vertline. of the steering angle increases from 0, and the adaptation is a fixed value of 1 at 20 degrees. Therefore, as described above, when the vehicle is running on a moderate and long curve with a large curvature of an expressway, the steering wheel is steered at a steering angle according to the curvature of the road, and the steering assist amount of the vehicle can be increased to make the steering force lighter to some extent. However, when the vehicle is running on a road with a cant (inclination in width direction of the road), the absolute value .vertline.ha.vertline. of steering angle is about 1 to 3 degrees, and the steering holding adaption is small. As a result, the steering assist amount of the vehicle is small and the steering force is heavy. Thus, the driver is required to use a high amount of steering (steering holding) force, which is a heavy burden to the driver.
To eliminate such problems, it is considered to use steering angular velocity to detect the steering holding condition as described in Japanese Patent Publication 3-35145 (USP 4530414, EP 90294).
In this prior art, a case where the steering wheel is turned to cause the vehicle to be released from a straightforward running and the steering angular velocity decreases to below a predetermined value is determined as a steering holding condition, and control for steering holding is made. Since the steering holding condition is determined from a low steering angular velocity, influence of the vehicle speed on the determination of the steering holding condition can be eliminated.
However, in this method, even when the driver slowly turns the steering wheel, a steering holding condition may be determined from the low steering angular velocity, and an appropriate assist amount for steering cannot be set.
Therefore, all object of the present invention is to provide a control system and control method for controlling a power steering apparatus which appropriately detects the steering holding condition and provides an optimum steering characteristic.
Further, the above-described fuzzy logic control type electronically controlled power steering apparatus was not able to obtain a sufficient stability of steering during high speed running of the vehicle. That is, in the above electronically controlled power steering apparatus, the number of changing times of steering angular velocity in a unit time is applied as one of the factors for setting the level of the steering holding condition so that a steering holding adaptation (level of steering holding condition) is a fixed value of 1 when the number of changing times is 0 to 2, decreases as the number of changing times increases from 2, and is a fixed value of 0 at the number of changing times of 3. Specifically, the steering angular velocity is determined from the steering angle detected by the steering angle sensor, a determination is made as to the steering angular velocity is in which of the three regions, a steering holding region, a left steering region, and a right steering region, and the number of times of phenomenon where the steering angular velocity changes to another region in 4 seconds is counted. Therefore, as described above, when the vehicle is running on a moderate and long curve with a large curvature of an expressway, the steering wheel is held at a steering angle according to the curvature, the number of times the steering angular velocity changes in 4 seconds is 0 and the adaptation is 1, and the steering assist amount of the vehicle is increased to decrease the steering force to some extent.
From such a steering holding condition while the vehicle is running at a high speed, when the driver steers the steering wheel by some amount for changing the lanes or the like, the steering force characteristic is desired to present a somewhat, heavy steering force to prevent oversteering. In the above power steering apparatus, the steering assist amount is increased and the steering force is made somewhat light in the steering holding condition during high speed running. When the steering wheel is steered by a predetermined amount for changing lanes or the like, the steering adaptation is decreased because the number of times that the steering angular velocity changes, and the steering assist amount is decreased to make steering heavier. However, in this power steering apparatus, the number of times that the steering angular velocity changes, must be counted at least for 4 seconds to decrease the steering holding adaptation, resulting in poor response. That is, when the driver momentarily operates the steering wheel to dodge a disturbance on the expressway or the like, the steering assist amount cannot be quickly decreased to make steering heavier. This is because this would result in unstable steering of the vehicle.
It is a primary object of the present invention to provide a control system and control method for controlling a power steering apparatus by which an optimum steering characteristic can be obtained in response to a running condition of the vehicle, eliminating the problems described above.